1. Technical Field
This invention relates to mobile satellite communications systems and, more particularly, to a mobile handset tracking and paging system for a mobile communications system in which the mobile handsets communicate directly with sub-geosynchronous satellites.
2. Discussion
Geosynchronous satellite constellations are less than ideal for use in mobile communications systems in which mobile handsets communicate directly with the orbiting satellites. Launching the satellites into geosynchronous orbit is more costly than lower orbits. Mobile handset transmitter power requirements become greater with increasing orbital distances from the earth. Transmission delays are prohibitively long, especially when the mobile handset communicates with another mobile handset, requiring transmission to geosynchronous orbit and back twice.
Proposed mobile satellite communications systems include mobile handsets which communicate directly with a sub-geosynchronous satellite constellation including a plurality of satellites. Such sub-geosynchronous satellites have lower launch costs, relatively low-power transmitters, and acceptable transmission delays. Each satellite handles a coverage region and divides the coverage region into adjacent subregions. The coverage region of one satellite slightly overlaps the coverage region of another satellite such that earth landmasses are completely covered by the sub-geosynchronous satellite constellation. Each satellite includes antennas which can generate multiple focused beams which may slightly overlap to handle the coverage region of the satellite. If the satellite is orbiting at sub-geosynchronous altitudes, areas covered by each satellite are changing, in contrast with geosynchronous satellite constellations in which coverage regions remain relatively fixed. In addition, mobile users are also changing position with time.
When a first fixed or mobile handset user wishes to initiate a call with a second mobile handset user, the mobile satellite communications system must page the second mobile handset using individual focused beams covering every subregion in which the second mobile handset user might be located. If the mobile communications system is ultimately to be global in scope, it must be assumed that the second mobile handset user might potentially be located anywhere on the earth. Accordingly, it may be possible that more than one satellite would need to be used to page a mobile user.
Paging the second mobile handset user with all of the individual focused beams for one or more satellites in the sub-geosynchronous satellite constellation would be extremely wasteful of both satellite power and bandwidth. Even in a more restricted mobile communications systems, paging the mobile user with most or all of the individual focused beams covering a large region, such as a country or state, would similarly be wasteful of both satellite power and bandwidth. Requiring the mobile user to transmit a signal to a satellite on a periodic basis to allow the communication system to readily locate the mobile handset is also wasteful. As battery-powered mobile handsets are quickly becoming the cellular phone of choice, unnecessary transmissions could substantially reduce available power which is needed by the user for normal telephone communication.
Therefore, a system which can track the mobile user with sufficient accuracy to reduce the number of individual focused beams used to page the mobile user, and consequently reducing bandwidth allocated to paging, is desirable.